Page 777 - Veterinary Immunology, 10th Edition

the vaccine viruses may themselves cause disease or persistent
VetBooks.ir infection (due to residual virulence). Killed organisms, in contrast,

act as exogenous antigens. They commonly stimulate type 2
+
responses dominated by CD4 T cells and antibodies. This may not
be the most appropriate response to some organisms, but it may be
safer. Dendritic cells respond in a different fashion to live and killed
bacteria.
The practical advantages and disadvantages of vaccines

containing living or killed organisms are well demonstrated in the
vaccines available against Brucella abortus in cattle. B. abortus is a
cause of abortion in cattle, and vaccination has been used
historically to control the disease. Brucella infections are best

controlled by type 1 immune responses involving macrophage
activation, the production of IFN-γ by Th1 cells, and cytotoxic CD8 +
T cells. A vaccine containing a living attenuated strain of B. abortus
is required to induce these responses and control this infection.

Older attenuated Brucella vaccines, especially strain 19, caused a
lifelong immunity in cows and successfully prevented abortion.
Unfortunately, strain 19 vaccine also caused systemic reactions:
swelling at the injection site, high fever, anorexia, listlessness, and a

drop in milk yield. Strain 19 could cause abortion in pregnant cows,
orchitis in bulls, and undulant fever in humans. To eradicate
brucellosis, serological tests are used to identify infected animals,
and strain 19 causes an antibody response that is difficult to

distinguish from a natural infection.
Because of the disadvantages associated with the use of strain 19,
considerable efforts have been made to find a better alternative.
Unfortunately, killed vaccines (strain 45/20) protected cattle for less

than 1 year. An attenuated strain of B. abortus called RB-51 has been
used in cattle in the United States. This is a rough mutant that fails
to produce the lipopolysaccharide O antigen. It does produce a
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strong Th1 response with production of IFN-γ and CD8 cytotoxic
cells. Unlike strain 19, it does not induce false-positive results in the
standard diagnostic tests such as card agglutination, complement
fixation, or tube agglutination. It is therefore possible to distinguish
between vaccinated and infected cattle. RB-51 is less pathogenic for
cattle than strain 19, and it is not shed in nasal secretions, saliva, or

urine. RB-51 will not cause abortion in pregnant cattle. It will,

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